Bearer path optimization

ABSTRACT

Call control for originating and terminating calls in a visited circuit-switched subsystem (CS) or home multimedia subsystem (MS) as well as transferring calls between the visited CS and the home MS may be anchored at a continuity control function (CCF) in the home MS. Call signaling for the call may be passed through the CCF. When the user element is homed to the home MS and served by the visited CS, the bearer path for the call is established based on the relative proximity of the home MS and the visited CS. When a local MS is more proximate to the visited CS, the bearer path may be routed through a gateway in the local MS, instead of through the gateway in the home MS. When the home MS is sufficiently proximate to the visited CS, the bearer path is routed through the gateway in the home MS.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication Ser. No. 60/692,638, filed in the U.S. Patent and TrademarkOffice on Jun. 21, 2005, the disclosure of which is incorporated hereinby reference in its entirety. This application is related to U.S.utility patent application Ser. No. 11/378,776 filed on Mar. 17, 2006,the disclosure of which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to communications, and in particular toproviding a centralized control function for supporting calls overcircuit-switched subsystems and multimedia subsystems as well asoptimizing the bearer path for the calls while maintaining centralizedcontrol.

BACKGROUND OF THE INVENTION

Packet communications have evolved to a point where voice sessions, orcalls, can be supported with essentially the same quality of service asthat provided by circuit-switched communications. Packet communicationsare generally supported over packet subsystems, which were initiallysupported by local area networks, but are now supported by wirelesslocal area networks (WLANs). Using WLAN access, user elements cansupport voice sessions using packet communications while movingthroughout the WLAN. As such, WLAN access provides users the samefreedom of movement within a WLAN as cellular access provides userswithin a cellular environment.

In many instances, the coverage areas provided by WLANs and cellularnetworks are complementary. For example, a WLAN may be establishedwithin a building complex in which cellular coverage is limited. Giventhe localized nature of WLAN coverage, cellular networks could bridgethe coverage gaps between WLANs. Unfortunately, WLAN access technologyis independent of cellular access technology. Cellular networksgenerally support circuit-switched communications, and WLANs supportpacket communications. As such, user elements have been developed tosupport both cellular and WLAN communications using differentcommunication interfaces. With these user elements, users can initiateand receive calls via the cellular network and WLAN using the respectivecommunication interfaces.

In co-pending U.S. patent application Ser. No. 11/378,776, filed Mar.17, 2006, and entitled CIRCUIT-SWITCHED AND MULTIMEDIA SUBSYSTEM VOICECONTINUITY, which is incorporated herein by reference, applicant hasproposed moving a user element's service control, including callcontrol, from a cellular network to a multimedia subsystem (MS), such asthe Internet Protocol (IP) Multimedia Subsystem (IMS). As such, callcontrol is provided by the MS irrespective of whether the user elementis using cellular or WLAN access for the call. For clarity andconciseness, a cellular network providing circuit-switchedcommunications is referred to as circuit-switched subsystem (CS), and aWLAN providing packet communications is assumed to be part of orassociated with the MS. In general, wireless communication techniqueshaving relatively limited range, such as WLAN techniques, are referredto as local wireless communication techniques. Thus, local wirelesscommunication techniques support packet-based communications, whereincellular communication techniques will generally supportcircuit-switched communications. Further, the wireless access for localwireless techniques are of a limited range with respect to cellularaccess techniques.

Call control for originating and terminating calls in the CS or MS aswell as transferring calls between the CS and MS is anchored at acontinuity control function (CCF) in the MS. All call signaling for thecall is passed through the CCF. The CCF is a service provided in theuser element's home MS and anchors the user element's active CS callsand MS sessions to enable mobility across the CS and MS whilemaintaining CS calls or MS sessions.

For CS calls, the CCF operates to anchor the bearer path for callsoriginated or terminated through the CS by the user element at a mediagateway, which is controlled by a media gateway controller of the homeMS. The CCF employs Third Party Call Control function to provide callcontrol in the CS. For MS calls, the CCF provides call control byinteracting with the user element and a remote endpoint to establish abearer path directly between the user element and the remote endpointthrough the MS. The CCF is addressable using public service identities(PSI). In the CS, a directory number associated with the CCF is used forrouting call signaling messages within the CS. In the MS, a uniformresource location (URL) associated with the CCF is used for routing callsignaling messages within the MS. In the following description, 3GPP TS24H.008 (DTAP) is used in the CS, while the Session Initiation Protocol(SIP) is used in the MS to effect origination, termination, and transferof calls. Those skilled in the art will recognize other applicable anduseful protocols as substitutes for DTAP and SIP.

Turning now to FIG. 1, a communication environment 10 is illustratedwhere a home MS 12H and a visited CS 14 support communications for auser element 16. The user element 16 includes a CS client 18 and an MSclient 20, which are configured to support circuit-switchedcommunications via the visited CS 14 as well as packet communicationsvia the home MS 12H, respectively. For communications within the visitedCS 14, a visited mobile switching center (VMSC) 22 will supportcircuit-switched communications for the user element 16. The VMSC 22 mayinteract with the home MS 12H via a media gateway controller (MGC) 24Hand an associated media gateway (MG) 26H, both of which are affiliatedwith the MS 12H.

The home MS 12H may include various functions or entities, including aninterrogating and serving call/session control function (I/S-CSCF) 28, aCCF 30, an application server (AS) 32, and a home subscriber service(HSS) 34. Notably, the interrogating CSCF provides the standard I-CSCFfunctions and the serving CSCF provides standard S-CSCF functions. Thesefunctions are represented in the I/S-CSCF 28 for conciseness. Further,the HSS 34 may have a presence in both the visited CS 14 and the home MS12H. The HSS 34 may include a home location resource component for homeCS. Call/session control functions (CSCFs) in the home MS 12H generallyact as SIP proxies and provide various functions in association withcall control, as will be appreciated by those skilled in the art. Inoperation, an interrogating CSCF (I-CSCF) may interact with the HSS 34to identify the serving CSCF (S-CSCF), which will be assigned to supporta given user element. The HSS 34 may maintain an association between auser element 16 and a particular CCF 30 that is assigned to the userelement 16. As such, the HSS 34 will assist in identifying a servingCSCF for the user element 16, as well as keep an association between aparticular CCF 30 and the user element 16. The CCF PSI for the userelement 16 may be provisioned in the user element 16 to enable the userelement 16 to initiate transfers and the like controlled by the CCF 30.Alternatively, the CCF PSI may be transferred to the user element 16upon network registration.

Depending on whether the user element 16 is registered in the home MS12H, different techniques may be used to access the home MS 12H. Whenthe user element 16 is registered in the home MS 12H, the user element16 will have an S-CSCF assigned to it, and will use that S-CSCF toaccess the CCF 30. When the user element 16 is not registered in thehome MS 12H, a temporary S-CSCF may be assigned to the user element 16,and the temporary S-CSCF will be used to access the CCF 30.

The application servers 32 may be invoked and placed within the callsignaling path to implement any number of features or services. When aparticular application service provided by an application server 32 isinvoked, all signaling for the associated call or session is passedthrough the application service, which has the opportunity to processcall signaling messages as necessary to implement the desired service.Notably, the CCF 30 acts like a service, and as such, the I/S-CSCF 28will operate to pass all call signaling messages for the call throughthe CCF 30, thereby allowing the CCF 30 to act as an anchor for thecall.

In FIG. 1, the user element 16 is engaged in a call supported by the CSclient 18 and controlled by the CCF 30. As such, call signaling for thecall passes through the VMSC 22, media gateway controller 24H, I/S-CSCF28, CCF 30, and perhaps application server 32, if a service is invoked,on its way toward a remote endpoint 36. Notably, the access signalingleg, which is provided by the visited CS 14, is anchored at the CCF 30and extends through the I/S-CSCF 28, media gateway controller 24H, theVMSC 22, and CS client 18 of the user element 16. The remote signalingleg toward the remote endpoint 36 is anchored in the CCF 30 and extendsthrough the I/S-CSCF 28 and the application server 32. In thisconfiguration, the CCF 30 can maintain control of the call and provideany necessary call processing during the call. Further, if a calltransfer is required, the CCF 30 maintains the remote signaling leg andestablishes a new access signaling leg.

The bearer path for the call illustrated in FIG. 1 extends from the CSclient 18 through the VMSC 22 and media gateway 26H on its way towardthe remote endpoint 36. Notably, the media gateway controller 24Hcooperates with the media gateway 26H, such that a circuit-switchedconnection may be established between the media gateway 26H and the CSclient 18 via the VMSC 22. The packet session may be established for thecall from the media gateway 26H through the home MS 12H toward theremote endpoint 36.

With reference to FIG. 2, a call supported by the MS client 20 of theuser element 16 is represented. Notably, the call does not extendthrough the visited CS 14, and will not employ the services of the VMSC22, media gateway controller 24H, or media gateway 26H. Instead, the MSclient 20 will support call signaling directly with the home MS 12H, andin particular with the CCF 30 via a serving-CSCF (S-CSCF) 40. Notably,the I/S-CSCF 28 and the S-CSCF 40 may represent the same CSCF ordifferent CSCFs, depending on how the user element 16 registers with thehome MS 12H.

As illustrated, call signaling is anchored in the CCF 30, wherein anaccess signaling leg is provided between the CCF 30 and the MS client 20via the S-CSCF 40. A remote signaling leg is supported between theremote endpoint 36 and the CCF 30 via the S-CSCF 40 and any desiredapplication servers 32 that may provide additional services inassociation with the call. The bearer path will extend from the MSclient 20 toward the remote endpoint 36 via the home MS 12H, withouttraveling through the visited CS 14 (FIG. 1). Again, the CCF 30 anchorsthe call, such that if transfer is required, the remote signaling legtoward the remote endpoint 36 can be maintained, while the accesssignaling leg may be changed to facilitate the transfer from the home MS12H to the visited CS 14. For transfer of calls between the visited CS14 and the home MS 12H, the access signaling legs illustrated in FIGS. 1and 2 will be changed to support the transfer, while the remotesignaling leg is maintained by the CCF 30.

When the user element 16 is originating a call, the CCF 30 appears as aservice provided by an application server, such as the applicationserver 32. The CCF 30 may be invoked as the first service in a chain ofservices. When the user element 16 is terminating a call, the CCF 30 isinvoked as the last service in a chain of services. By locating the CCF30 with respect to the other services in this manner, other applicationsassociated with the call are anchored by the CCF 30 as part of theremote signaling leg of the call, and are therefore not impacted bytransfers affecting the access signaling leg.

The MSISDN or other user element identifier is owned and controlled bythe home MS 12H to enable anchoring of incoming calls intended for theuser element 16 at the CCF 30. Incoming calls destined for the userelement 16 and originated from the visited CS 14, the public switchedtelephone network (PSTN), or other MS can be anchored at the CCF 30 bysetting up routing functions at the originating service nodes, such thatincoming calls intended for the user element 16 are delivered to thehome MS 12H. As such, the CCF 30 can take the necessary steps to findthe user element 16 and route the call to the user element 16, even ifthe user element 16 is in the visited CS 14 when the call arrives.

As indicated, the HSS 34 may store filter criteria associated with theCCF 30 as part of the user element's subscription profile. The CCFfilter criteria is downloaded to the currently assigned S-CSCF (28 or40) as part of the initial filter criteria to use when the user element16 registers with the home MS 12H. This filter criteria is generallyexecuted at the S-CSCF 40 (or 28) upon initiation of a call or sessionfrom the user element 16 or upon receipt of an incoming session intendedfor the user element 16. These filter criteria will instruct the S-CSCF40 (or 28) to invoke the CCF 30 to control at least the bearer path forthe call or session.

As illustrated in FIG. 1, when the user element 16 is being served bythe visited CS 14, which is located a relatively long distance from thehome MS 12H, routing the bearer path to the media gateway 26H in thehome MS 12H may induce some backhaul inefficiencies. As such, the bearerpath may be indirectly routed between the user element 16 and the remoteendpoint 36 through the media gateway 26H instead of being more directlyrouted through the visited CS 14 and more local networks.

Accordingly, there is a need for a technique to effectively supportcalls for the user element 16 over both the visited CS 14 and the homeMS 12 at the CCF 30. In conjunction, there is a further need toconstruct a more efficient bearer path between the user element 16 andthe remote endpoint 36 when the visited CS 14 is located a relativelylong distance from the home MS 12H.

SUMMARY OF THE INVENTION

For the present invention, call control for originating and terminatingcalls in a visited CS or home MS as well as transferring calls betweenthe visited CS and the home MS may be anchored at a CCF in the home MSas described above. The user element is homed to the home MS. When theCCF is invoked, call signaling for the call is passed through the CCF.

When the user element is served by the visited CS, the bearer path forthe call is established based on the relative proximity of the home MSand the visited CS. When a local MS is more proximate to the visited CS,the bearer path may be routed through a gateway in the local MS, insteadof through the gateway in the home MS. When the home MS is sufficientlyproximate to the visited CS or more proximate to the visited CS than thelocal MS, the bearer path is routed through the gateway in the home MS.The present invention reduces the length of the time divisionmultiplexed (TDM) portion of the bearer path by selectively invoking thegateway in the local MS based on the relative proximities of the localMS and the home MS to the visited CS. As such, the TDM portion of thebearer path extending from the visited CS may be selectively routed tothe gateway in the local MS to avoid an unduly long run to the gatewayin the home MS, when there is a local MS available.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a communication environment illustrating circuit-switchedsubsystem access for a user element.

FIG. 2 is a communication environment illustrating multimedia subsystemaccess for a user element.

FIG. 3 is a communication environment illustrating circuit-switchedsubsystem access for a user element according to a first embodiment ofthe present invention.

FIGS. 4A-4C show a communication flow illustrating originating a callvia the circuit-switched subsystem according to one embodiment of thepresent invention.

FIG. 5 is a communication environment illustrating circuit-switchedsubsystem access for a user element according to a second embodiment ofthe present invention.

FIGS. 6A-6C show a communication flow illustrating originating a callvia the circuit-switched subsystem according to another embodiment ofthe present invention.

FIG. 7 is a communication environment illustrating circuit-switchedsubsystem access for a user element according to a third embodiment ofthe present invention.

FIGS. 8A and 8B show a communication flow illustrating terminating anincoming call via the circuit-switched subsystem according to oneembodiment of the present invention.

FIG. 9 is a block representation of a service node according to oneembodiment of the present invention.

FIG. 10 is a block representation of a user element according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

For the present invention, call control for originating and terminatingcalls in the visited CS or home MS as well as transferring calls betweenthe visited CS and the home MS may be anchored at the CCF in the home MSas described above. The user element is homed to the home MS. When theCCF is invoked, call signaling for the call is passed through the CCF.

When the user element is served by the visited CS, the bearer path forthe call is established based on the relative proximity of the home MSand the visited CS. When a local MS is more proximate to the visited CS,the bearer path may be routed through a gateway in the local MS, insteadof through the gateway in the home MS. When the home MS is sufficientlyproximate to the visited CS or more proximate to the visited CS, thebearer path is routed through the gateway in the home MS. The presentinvention reduces the length of the time division multiplexed (TDM)portion of the bearer path by selectively invoking the gateway in thelocal MS based on the relative proximities of the local MS and the homeMS to the visited CS. As such, the TDM portion of the bearer pathextending from the visited CS may be selectively routed to the gatewayin the local MS to avoid an unduly long run to the gateway in the homeMS, when there is a local MS available.

With particular reference to FIG. 3, the bearer path, access signalingleg, and remote signaling leg for a call originated from the userelement 16 to the remote endpoint 36 are shown according to oneembodiment of the present invention. In this embodiment, the userelement 16 is homed to the home MS 12H and roaming into the visited CS14. The remote endpoint 36 is currently being served by the home MS 12H.When the visited CS 14 is not proximate to the home MS 12H, callsignaling is still anchored at the CCF 30 in the home MS 12H. However,the TDM portion of the bearer path is not established to the mediagateway 26H in the home MS 12H as provided in FIG. 1. Instead, the TDMportion of the bearer path is established to a media gateway 26L in alocal MS 12L, which is more proximate to the visited CS 14 than the homeMS 12H. The packet portion of the bearer path is established from themedia gateway 26L to the remote endpoint 36 via the home MS 12H.

The access signaling leg to the CCF 30 is established via the VMSC 22 inthe visited CS 14, a media gateway controller 24L of the media gateway26L in the local MS 12L, an interrogating CSCF (I-CSCF) 38L in the localMS 12L, and the I/S-CSCF 28 in the home MS 12H. As such, the presentinvention establishes the bearer path through the local MS 12S, which isproximate to the visited CS 14, instead of through the home MS 12H toprovide more efficient bearer path routing when the home MS 12H is notproximate to the visited CS 14. Control of the call is maintained at theCCF 30.

With reference to FIGS. 4A through 4C, a communication flow is providedto illustrate origination of a call from the CS client 18 of the userelement 16 via the visited CS 14 according to one embodiment of thepresent invention. The user element 16 is homed to the home MS 12H androaming into the visited CS 14, and the remote endpoint 36 is currentlybeing served by the home MS 12H of the user element 16. Initially, theCS client 18 will send a call setup message to its supporting VMSC 22 inthe visited CS 14 (step 100). The call setup message will identify thedirectory number of the remote endpoint 36. The VMSC 22 is provisionedto initially route the call through the more proximate local MS 12Linstead of the more distant home MS 12H. As such, the VMSC 22 will sendthe media gateway controller 24L in the local MS 12L an IntegratedServices User Part (ISUP) Initial Address Message (IAM) addressing arouting number that identifies the remote endpoint 36, as well asincluding the CS PSI for the CCF 30, which is located in the home MS 12H(step 102). The VMSC 22 will also provide a Call Proceeding message backto the CS client 18 of the user element 16 to indicate the call isprogressing (step 104). This exchange results in a TDM-based CS bearerpath being established from the CS client 18 to the media gateway 26L inthe local MS 12L via the VMSC 22 (step 106). The media gatewaycontroller 24L may act as a user agent on behalf of the user element 16.

Upon receiving the IAM from the VMSC 22, the media gateway controller24L will send an Invite to the I-CSCF 38L in the local MS 12L (step108). When generating the Invite, the media gateway controller 24L willidentify the MS-based PSI for the CCF 30 in light of the CS-based PSIreceived in the IAM. The MS-based PSI for the CCF 30 is the address towhich the Invite is intended. The Invite will also identify the userelement 16 as the originator of the call and identify the remoteendpoint 36 as the destination for the call. The Invite may alsoindicate that the call was originated through the CS 14.

Upon receipt of the Invite, the I-CSCF 38L will identify the home MS 12Hof the user element 16 and send an Invite to the I/S-CSCF 28 in the homeMS 12H to initiate establishment of a packet-based MS bearer path towardthe remote endpoint 36 for which the call is intended from the mediagateway 26L (step 110). The I/S-CSCF 28 will recognize the need toinvoke the CCF 30 for the call, and will send the Invite to the CCF 30(step 112), which may invoke a back-to-back user agent (B2BUA) and thentake the necessary steps to complete the call (step 114). As such, theCCF 30 will send an Invite back to the I/S-CSCF 28 to complete the call(step 116). The Invite will now include the address of the remoteendpoint 36 or a supporting node with which a packet session can beestablished. The Invite will identify the media gateway controller 24Lof the media gateway 26L as the other endpoint for the packet sessionthat will support the call. The I/S-CSCF 28 will then send the Invitetoward the remote endpoint 36 (step 118). At this point, the traditionalsession message exchange between the remote endpoint 36 and the mediagateway controller 24L in the local MS 12L will take place through theCCF 30, perhaps the I-CSCF 38L, and the I/S-CSCF 28 to prepare therespective remote endpoint 36 and media gateway 26L to support the MSbearer leg (step 120).

In the meantime, the I/S-CSCF 28 may receive various signaling back fromthe remote endpoint 36, such as a 180 Ringing message (step 122). The180 Ringing message indicates that the call is being presented to theremote endpoint 36. The I/S-CSCF 28 will route all signaling messagesthrough the CCF 30, and as such, the 180 Ringing message is sent to theCCF 30 (step 124), which will forward the message back to the I/S-CSCF28 (step 126). The I/S-CSCF 28 will send the 180 Ringing message to themedia gateway controller 24L in the local MS 12L directly or via theI-CSCF 38L (steps 128 and 130). After a Provisional Acknowledgement(PRACK) and 200 OK message exchange between the media gateway controller24L and the remote endpoint 36 through the CCF 30 (step 132), the MSbearer path is effectively established (step 134), which means that themedia gateway 26 and the remote endpoint 36 can send packets back andforth in association with the call.

The media gateway controller 24L will send an Address Complete Message(ACM) to the VMSC 22 (step 136), which will send an Alerting message tothe user element 16 to indicate that call is being presented to theremote endpoint 36 (step 138). When the call is answered by the remoteendpoint 36, the l/S-CSCF 28 will receive a 200 OK message (step 140)and route the 200 OK message to the CCF 30 (step 142). The CCF 30 willprocess the message if necessary, and then send the message back to theI/S-CSCF 28 (step 144), which will forward the 200 OK message to themedia gateway controller 24L directly or via the I-CSCF 38L (steps 146and 148). The media gateway controller 24L will send an Answer Message(ANM) to the VMSC 22 (step 150), which will send a Connect message tothe CS client 18 (step 152) to indicate that the call has been answered.To complete the call, the media gateway controller 24L will provideappropriate signaling to the media gateway 26L, as well as send anacknowledgement (ACK) message back to the I/S-CSCF 28 directly or viathe I-CSCE 38L (steps 154 and 156). The I/S-CSCF 28 will again forwardthe ACK to the CCF 30 (step 158), which will forward the message back tothe I/S-CSCF 28 (step 160). The I/S-CSCF 28 will then send the ACKtoward the remote endpoint 36 (step 162).

At this point, a CS/MS bearer path is established between the CS client18 of the user element 16 and the remote endpoint 36 via the mediagateway 26 (step 164). The TDM portion of the CS/MS beater path extendsfrom the VMSC 22 to the media gateway 26L in the local MS 12L instead ofthe media gateway 26H in the home MS 12H to provide a shorter, moreefficient bearer path. However, the call signaling associated with thecall will be routed through the CCF 30. Notably, the back-to-back useragent invoked by the CCF 30 is the function that represents an endpointfor signaling associated with the remote signaling leg as well as anendpoint for the access signaling leg. The back-to-back user agent willprovide any necessary processing or filtering and then relay messagesover the respective access and remote signaling legs.

In FIG. 5, the bearer path, access signaling leg, and remote signalingleg for a call originated from the user element 16 to the remoteendpoint 36 are shown. In this embodiment, the user element 16 is homedto the home MS 12H and roaming into the visited CS 14. The remoteendpoint 36 is being served directly or indirectly by the VMSC 22instead of the home MS 12H. When the visited CS 14 is not proximate tothe home MS 12H, call signaling is still anchored at the CCF 30 in thehome MS 12H. However, the TDM portion of the bearer path is notestablished through the media gateway 26H in the home MS 12H and loopedback to the VMSC 22. Instead, the TDM portion of the bearer path isestablished through the media gateway 26L in the local MS 12L, which ismore proximate to the visited CS 14 than the home MS 12H, and loopedback to the VMSC 22.

The access signaling leg to the CCF 30 is established via the VMSC 22 inthe visited CS 14, the media gateway controller 24L of the media gateway26L in the local MS 12L, an interrogating CSCF (I-CSCF) 38L in the localMS 12L, and the I/S-CSCF 28 in the home MS 12H. The remote signaling legtoward the remote endpoint 36 is established from the CCF 30 via theI/S-CSCF 28, a border gateway control function (BGCF) 40H in the home MS12H, a BGCF 40L in the local MS 12L, the media gateway controller 24Land the VMSC 22. As such, the present invention establishes the bearerpath through the media gateway 26L in the local MS 12L, which isproximate to the visited CS 14 instead of through the media gateway 26Hin the home MS 12H, to provide more efficient bearer path routing whenthe home MS 12H is not proximate to the visited CS 14. As illustrated,the entire bearer path is TDM and routed in an efficient fashion, yetcontrol of the call is maintained by the CCF 30.

With reference to FIGS. 6A through 6C, a communication flow is providedto illustrate origination of a call from the CS client 18 of the userelement 16 via the visited CS 14 according to one embodiment of thepresent invention. The user element 16 is homed to the home MS 12H androaming into the visited CS 14. The remote endpoint 36 is being serveddirectly or indirectly by the VMSC 22 instead of the home MS 12H.Initially, the CS client 18 will send a call setup message to itssupporting VMSC 22 in the visited CS 14 (step 200). The call setupmessage will identify the directory number of the remote endpoint 36.The VMSC 22 is provisioned to initially route the call through the moreproximate local MS 12L instead of the more distant home MS 12H. As such,the VMSC 22 will send the media gateway controller 24L in the local MS12L an Integrated Services User Part (ISUP) Initial Address Message(IAM) addressing a routing number that identifies the remote endpoint36, as well as including the CS PSI for the CCF 30, which is located inthe home MS 12H (step 202). The VMSC 22 will also provide a CallProceeding message back to the CS client 18 of the user element 16 toindicate the call is progressing (step 204). This exchange results in aTDM-based CS bearer path being established from the CS client 18 to themedia gateway 26L in the local MS 12L via the VMSC 22.

Upon receiving the IAM from the VMSC 22, the media gateway controller24L will send an Invite to the I-CSCF 38L in the local MS 12L (step206). When generating the Invite, the media gateway controller 24L willidentify the MS-based PSI for the CCF 30 in light of the CS-based PSIreceived in the IAM. The MS-based PSI for the CCF 30 is the address towhich the Invite is intended. The Invite will also identify the userelement 16 as the originator of the call and identify the remoteendpoint 36 as the destination for the call. The Invite may alsoindicate that the call was originated through the CS 14.

Upon receipt of the Invite, the I-CSCF 38L will identify the home MS 12Hof the user element 16 and send an Invite to the I/S-CSCF 28 in the homeMS 12H to initiate establishment of the call to the remote endpoint 36(step 208). The I/S-CSCF 28 will recognize the need to invoke the CCF 30for the call, and will send the Invite to the CCF 30 (step 210), whichmay invoke a back-to-back user agent (B2BUA) and then take the necessarysteps to complete the call (step 212). As such, the CCF 30 will send anInvite back to the I/S-CSCF 28 to complete the call (step 214). TheInvite will include the address, which may be a directory number, of theremote endpoint 36 to which the call is directed and be sent to themedia gateway controller 24L via the BGCF 40H in the home MS 12H and theBGCF 40L in the local MS 12L (steps 216, 218, and 220).

The media gateway controller 24L will then initiate a TDM-based calltoward the remote endpoint 36 by sending an IAM toward the VMSC 22 (step222), which will send an IAM toward the VMSC 22 via a gateway MSC (notshown) currently serving the remote endpoint 36 (step 224). In response,the VMSC 22 will receive an ACM (step 226) and send an ACM to the mediagateway controller 24L to indicate that the call is progressing (step228). The media gateway controller 24L will initiate a 180 Ringingmessage, which is routed along the remote signaling leg to the CCF 30via the BGCF 40L, BGCF 40H, and I/S-CSCF 28 (steps 230, 232, 234, and236). The CCF 30 will forward the 180 Ringing message along the accesssignaling leg back to the media gateway controller 24L via the I/S-CSCF28 and I-CSCF 38L (steps 238, 240 and 242). In response, the mediagateway controller 24L will send an ACM to the VMCS 22 (step 244), whichwill send an alerting message to the CS client 18 of the user element 16to indicate that the call is being presented to the remote endpoint 36(246).

Once the call is answered, an ACM is received by the VMSC 22 from thegateway or MSC (not shown) serving the remote endpoint 36 (step 248).The VMSC 22 will send an ACM to the media gateway controller 24L (step250), which will route a 200 OK message toward the CCF 30 over theremote signaling leg via the BGCF 40L, BGCF 40H, and the I/S-CSCF 28(steps 252, 254, 256, and 258). The CCF 30 will forward the 200 OK alongthe access signaling leg back to the media gateway controller 24L viathe I/S-CSCF 28 and I-CSCF 38L (steps 260, 262, and 264). In response,the media gateway controller 24L will send an ANM to the VMSC 22 (step266), which will send an alerting message to the CS client 18 of theuser element 16 to indicate that the call has been answered (step 268).During this process the media gateway controller 24L will instruct themedia gateway 26L to establish a TDM-based CS bearer path between theVMSC 22 and the gateway or MSC (not shown) serving the remote endpoint36 (step 270). The CS bearer path extends from the CS client 18 of theuser element 16 to the media gateway 26L through the VMSC 22 and on tothe remote endpoint 36 back through the VMSC 22. As such, the TMD-basedbearer path is not routed through the media gateway 26H in the home MS12H to improve routing efficiency; however, call signaling is routedthrough the home MS 12H to allow anchoring at the CCF 30.

In FIG. 7, the bearer path, access signaling leg, and remote signalingleg for a call terminated at the user element 16 from a call originatedfrom the remote endpoint 36 are shown. In this embodiment, the userelement 16 is homed to the home MS 12H and roaming into the visited CS14. The remote endpoint 36 is currently being served by the home MS 12H.When the visited CS 14 is not proximate to the home MS 12H, callsignaling is still anchored at the CCF 30 in the home MS 12H. However,the TDM portion of the bearer path is not established from the mediagateway 26H in the home MS 12H. Instead, the TDM portion of the bearerpath is established from the media gateway 26L in the local MS 12L,which is more proximate to the visited CS 14 than the home MS 12H, tothe VMSC 22. The packet portion of the bearer path is established to themedia gateway 26L from the remote endpoint 36 via the home MS 12H.

The access signaling leg from the CCF 30 toward the user element 16 isestablished from the CCF 30 via the I/S-CSCF 28, the BGCF 40H in thehome MS 12H, the BGCF 40L in the home MS 12L, the media gatewaycontroller 24L, and the VMSC 22. As such, the present inventionestablishes the bearer path through the media gateway 26L in the localMS 12L, which is proximate to the visited CS 14, instead of through themedia gateway 26H in the home MS 12H to provide more efficient bearerpath routing when the home MS 12H is not proximate to the visited CS 14.Control of the call is maintained at the CCF 30.

With reference to FIGS. 8A and 8B, a communication flow for terminatinga call to the user element 16 via the visited CS 14 is provided. In thisexample, assume that the call is initiated from a remote endpoint 36served by the home MS 12H. Further assume that the user element 16 ishomed to the home MS 12H and roaming in the visited CS 14. As such, anInvite from the remote endpoint 36 is delivered to the I/S-CSCF 28 inthe home MS 12H (step 300). The Invite will identify the address of theuser element 16. Applying the appropriate filter criteria to the Invitewill result in the I/S-CSCF 28 forwarding the Invite to the CCF 30 (step302). Again, the filter criteria may be obtained from the HSS 34, andwill direct the I/S-CSCF 28 to invoke the CCF 30 as a service requiredfor call signaling. The CCF 30 may also access information from the HSS34. The information in the HSS 34 may indicate that the user element 16is being served by the visited CS 14, and that calls should be routed tothe CS client 18 of the user element 16.

As such, the CCF 30 will invoke a back-to-back user agent and direct thesession to the visited CS 14 via the I/S-CSCF 28 (step 304).Accordingly, an Invite is sent back to the I/S-CSCF 28 (step 306), whichwill forward the Invite to the BGCF 40H in the home MS 12H (step 308).The BGCF 40H will forward the Invite to the BGCF 40L in the local MS 12L(step 310). The BGCF 40L will forward the Invite to the media gatewaycontroller 24L (step 312). The requisite session message exchange willthen take place between the media gateway controller 24L and the remoteendpoint 36 via the BGCF 40L, BGCF 40H, I/S-CSCF 28, and CCF 30 (step314). The media gateway controller 24L will then send an IAM to the VMSC22 in the visited CS 14 via an appropriate gateway mobile switchingcenter (not shown) (step 316). The VMSC 22 will send a Setup message tothe CS client 18 (step 318) to indicate that an incoming call is beingrouted to the user element 16. The CS client 18 of the user element 16will respond by sending a Call Confirmation message to the VMSC 22 (step320).

When the CS client 18 provides an alert to the user of the user element16, an Alerting message will also be sent back to the VMSC 22 toindicate that the call is being presented to the user (step 322). TheVMSC 22 will send an ACM to the media gateway controller 24L (step 324).In response, the media gateway controller 24L will send a 180 Ringingmessage to the I/S-CSCF 28 via the BGCF 40L and the BGCF 40H (steps 326,328, and 330). The I/S-CSCF 28 will send the 180 Ringing message to theCCF 30 (step 332). The CCF 30 will send the 180 Ringing message back tothe I/S-CSCF 28 (step 334), which will send the 180 Ringing messagetoward the remote endpoint 36 (step 336).

In this fashion, the signaling is routed through the CCF 30 by theI/S-CSCF 28. The remote endpoint 36 and the media gateway controller 24Lwill exchange the PRACK and 200 OK messages via the BGCF 40L, BGCF 40H,I/S-CSCF 28, and CCF 30 (step 338), wherein the packet-based MS bearerpath is established between the media gateway 26L and the remoteendpoint 36 (step 340). When the user of user element 16 answers theincoming call, the CS client 18 will send a Connect message to the VMSC22 (step 342), which will send an ANM toward the media gatewaycontroller 24L (step 344). At this point, the TDM-based CS bearer pathis established for the CS client 18 of the user element 16 between theVMSC 22 and the media gateway 26L (step 346).

In response, the media gateway controller 24L will send a 200 OK messageto the I/S-CSCF 28 via the BGCF 40L and the BGCF 40H (steps 348, 350,and 352). The I/S-CSCF 28 will send the 200 OK message to the CCF 30(step 354). The CCF 30 will send the 200 OK message back to the I/S-CSCF28 (step 356), which will send the 200 OK message toward the remoteendpoint 36 (step 358). Upon receiving an Acknowledgement (ACK)originated at the remote endpoint 36 (step 360), the media gatewaycontroller 24L will instruct the media gateway 26L in the local MS 12Lto effectively connect the CS bearer path and the MS bearer path to formthe CS/MS bearer path, which extends between the remote endpoint 36 andthe CS client 18 through the media gateway 26L in the local MS 12L andthe VMSC 22 (step 362). Notably, the TDM portion of the CS bearer pathextends to the media gateway 26L in the local MS 12L and not the mediagateway 26H in the home MS 12H.

With reference to FIG. 9, a service node 44 is provided according to oneembodiment of the present invention. The service node 44 may reside inthe MS 12 and include a control system 46 and associated memory 48 toprovide the functionality for any one or a combination of the following:the CCF 30, the I-CSCF 38, and the I/S-CSCF 28. The control system 46will also be associated with a communication interface 50 to facilitatecommunications with any entity affiliated with the MS 12 orappropriately associated networks.

With reference to FIG. 10, a block representation of a user element 16is provided. The user element 16 may include a control system 52 havingsufficient memory 54 to support operation of the CS client 18 and the MSclient 20. The control system 52 will cooperate closely with acommunication interface 56 to allow the CS client 18 and the MS client20 to facilitate communications over the CS 14 or the MS 12 as describedabove. The control system 52 may also be associated with a userinterface 58, which will facilitate interaction with the user. The userinterface 58 may include a microphone and speaker to facilitate voicecommunications with the user, as well as a keypad and display to allowthe user to input and view information.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A method of providing bearer path establishmentcomprising: receiving a Session Initiation Protocol (SIP) INVITE requestat an application server (AS) in a first multimedia subsystem providingcontinuity; in response to receiving the SIP INVITE request,establishing a signaling anchor point at the AS for an access signalingleg and a remote signaling leg for a call between a user element servedby a visited circuit-switched subsystem and a remote end-point; andsending a SIP message from the AS, the message effecting establishmentof at least part of a circuit-switched portion of a bearer path for thecall via a first gateway in a second multimedia subsystem.
 2. The methodof claim 1, wherein the SIP message is one of a SIP 200 ok message, aSIP provisional message, a SIP Provisional Acknowledgement message, or aSIP response message to the SIP INVITE request.
 3. The method of claim 1wherein the bearer path comprises a packet-based portion extending fromthe first gateway toward the remote endpoint.
 4. The method of claim 1wherein the user element comprises a circuit-switched client and amultimedia subsystem client and further comprising anchoring callssupported by the circuit-switched client and the multimedia subsystemclient in the first multimedia subsystem.
 5. The method of claim 1wherein the user element is homed to the first multimedia subsystem andbeing served by the visited circuit-switched subsystem.
 6. Anapplication server comprising: a memory for storing computer-executableinstructions; and a control system associated with the memory andconfigured to execute the computer-executable instructions to: receive aSession Initiation Protocol (SIP) INVITE request at the applicationserver (AS) in a first multimedia subsystem providing continuity; inresponse to receipt of the SIP INVITE request, establish a signalinganchor point at the AS for an access signaling leg and a remotesignaling leg for a call between a user element served by a visitedcircuit-switched subsystem and a remote end-point; and send a SIPmessage from the AS, the message effecting establishment of at leastpart of a circuit-switched portion of a bearer path for the call via afirst gateway in a second multimedia subsystem.
 7. The applicationserver of claim 6, wherein the SIP message is one of a SIP 200 okmessage, a SIP provisional message, a SIP Provisional Acknowledgementmessage, or a SIP response message to the SIP INVITE request.
 8. Theapplication server of claim 6 wherein the bearer path comprises apacket-based portion extending from the first gateway toward the remoteendpoint.
 9. The application server of claim 6 wherein the user elementcomprises a circuit-switched client and a multimedia subsystem clientand further comprising anchoring calls supported by the circuit-switchedclient and the multimedia subsystem client in the first multimediasubsystem.
 10. The application server of claim 6 wherein the userelement is homed to the first multimedia subsystem and being served bythe visited circuit-switched subsystem.
 11. A method of providing bearerpath establishment comprising: sending a Session Initiation Protocol(SIP) INVITE request from a first control function in a first multimediasystem; in response to the SIP INVITE request, effecting a signalinganchor point at an application server (AS) in a second multimedia systemfor an access signaling leg and a remote signaling leg for a callbetween a user element served by a visited circuit-switched subsystemand a remote end-point; and receiving a SIP message at the first controlfunction, the SIP message effecting establishment of at least part of acircuit-switched portion of a bearer path for the call via a firstgateway in the second multimedia subsystem.
 12. The method of claim 11,wherein the SIP message is one of a SIP 200 ok message, a SIPprovisional message, a SIP Provisional Acknowledgement message, or a SIPresponse message to the SIP INVITE request.
 13. The method of claim 11wherein the bearer path comprises a packet-based portion extending fromthe first gateway toward the remote endpoint.
 14. The method of claim 11wherein the user element comprises a circuit-switched client and amultimedia subsystem client and further comprising anchoring callssupported by the circuit-switched client and the multimedia subsystemclient in the first multimedia subsystem.
 15. The method of claim 11wherein the user element is homed to the first multimedia subsystem andbeing served by the visited circuit-switched subsystem.
 16. A controlfunction node comprising: a memory for storing computer-executableinstructions; and a control system associated with the memory andconfigured to execute the computer-executable instructions to: send aSession Initiation Protocol (SIP) INVITE request from a first controlfunction in a first multimedia system; in response to the SIP INVITErequest, effect a signaling anchor point at an application server (AS)in a second multimedia system for an access signaling leg and a remotesignaling leg for a call between a user element served by a visitedcircuit-switched subsystem and a remote end-point; and receive a SIPmessage at the first control function, the SIP message effectingestablishment of at least part of a circuit-switched portion of a bearerpath for the call via a first gateway in the second multimediasubsystem.
 17. The control function node of claim 16, wherein the SIPmessage is one of a SIP 200 ok message, a SIP provisional message, a SIPProvisional Acknowledgement message, or a SIP response message to theSIP INVITE request.
 18. The control function node of claim 16 whereinthe bearer path comprises a packet-based portion extending from thefirst gateway toward the remote endpoint.
 19. The control function nodeof claim 16 wherein the user element comprises a circuit-switched clientand a multimedia subsystem client and further comprising anchoring callssupported by the circuit-switched client and the multimedia subsystemclient in the first multimedia subsystem.
 20. The control function nodeof claim 16 wherein the user element is homed to the first multimediasubsystem and being served by the visited circuit-switched subsystem.